Researchers have found that clearing of forests, whether through logging or wildfires, can greatly increase the likelihood of large floods. A study that used data from forest regions in south-east Australia over many years revealed that areas that experience widespread deforestation can see their flood risk increase by up to 700%.

This means that a place that might have had a major flood once every 64 years could now face the same level of flooding every 8 years. The reason for this rise is not just because of heavier rain or changes in weather, but due to the loss of trees in key areas that collect and manage water. The study was published in Communications Earth & Environment.

Lost Forests, Rising Waters

Forests play an important role in controlling how rainwater flows across the land. The leaves and branches of trees catch rainwater, slowing it down and spreading it out before it hits the ground. Beneath the trees, a layer of organic material, like fallen leaves and branches, helps to soak up and hold water, which reduces the speed at which it moves into rivers and streams.

When these layers are gone, rainwater hits the ground directly and drains quickly into waterways, increasing the likelihood of rivers overflowing. Without the protection of the forest, areas that collect water become more prone to flooding, especially after heavy rains.

Deforestation Disaster

The study looked at data collected over more than 50 years from regions in south-east Australia, an area known for both bushfires and floods. The researchers focused on three major fire years: 2003, 2007, and 2009, when large parts of the forests were destroyed by wildfires. They compared flood levels after these fires with floods that happened under similar weather conditions but without major forest loss.

The researchers excluded years with known climate anomalies, such as El Niño or La Niña, which influence rainfall, to ensure a fair comparison. This allowed them to see the effect of forest loss on flooding, separate from any climate changes. Their results showed that the chance of a major flood in an area increased from roughly once every 64 years to once every 8 years after significant forest loss. This increase was seen in all the areas studied.

The study also investigated whether the fires made the soil less able to absorb water. While intense heat can make soil repel water, the study found that this was not the main cause of the increased flooding risk. Instead, the main factor was the loss of tree cover and the layer of organic matter, both of which help absorb and slow down rainwater.

Forest Loss and Flood Risk

The researchers also compared areas where the forest was completely burned with nearby areas that had only minor damage. Even under the same rainfall conditions, heavily deforested areas saw higher flood levels. In contrast, areas where trees remained intact were better at reducing the volume and speed of water entering the system. This comparison helped confirm that the forests themselves, not just the weather or landscape, were responsible for the increased flood risk.

Although the study was done in Australia, the researchers believe the results apply more widely. Forests around the world are being cleared due to human activity and climate change, which can lead to more frequent and intense wildfires. 

Why Is It Important

This highlights the need for careful planning when forests are cleared, whether for development, farming, or other uses. Areas that are upstream of towns, cities, or farmland should be handled with extra care, as deforestation in these regions can greatly increase the risk of damaging floods.

This study shows that when forests are lost, the risk to human lives and property from flooding can rise quickly and dramatically. By protecting forested areas that collect water, we can lower that risk and create more resilient communities for the future.

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